An advantage of a shaping process of this kind is that it can produce directly the final geometry of the required optical lens so that the latter is directly usable by the optician.
Another and more important advantage is that it produces only weak and even tension stresses within the optical lens.
In practise these internal tension stresses are only in the order of one tenth of those in an optical lens made in the conventional manner by injection molding.
However, regardless of the process by which they are shaped, thermoplastics synthetic material optical lenses have a further disadvantage in their mediocre resistance to abrasion.
It has therefore been proposed to coat at least one side of such lenses with a thermosetting synthetic material protection film which has good resistance to abrasion.
Various coating methods have been proposed, the diversity of which bears witness to the difficulty of this problem.
The most usual method is first to shape the optical lens to its final geometry, whether by injection molding or by natural thermal sagging, and then to apply the necessary thermosetting synthetic material coating composition or varnish to the optical lens, and finally to harden, i.e. polymerize the coating composition by heating or by radiation.
Apart from the fact that the thickness of the resulting film is not always easy to control, a drawback of this coating method is that by its very nature it introduces an additional stage of manufacture, necessarily being carried out at a different stage from that in which the optical lens itself is made.
To combine these two stages into one attempts have been made to form the optical lens to its final geometry when already coated with its protective film, the latter being at least partially hardened beforehand.
This is the case, for example, in French patent application number 77 20742 filed 6 Jul. 1977 (published number 2 358 256) and in U.S. Pat. Nos. 2,322,310, 2,481,809 and 2,640,227.
All these methods conventionally require the application of high pressures (in the order of 7 kg/cm.sup.2 to 300 kg/cm.sup.2) in a press and in an uncontrolled manner and therefore result in non-negligible internal tension stresses in the optical lens finally obtained.
When the protection film applied to the optical lens is totally polymerized before final shaping of the lens, the shaping almost inevitably causes cracking of the protection film because of the stresses occurring at the interface between it and the substrate it covers.
If the protection film is only partially polymerized before final shaping of the optical lens it is difficult to control its thickness at the end of the shaping process.
Finally, if the pressure that has to be used to shape the optical lens to its final shape is particularly high, it necessitates softening of the substrate beforehand, which is undesirable.